river hydrographs to find out discharge patterns of a particular drainage basin help predict...
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River Hydrographs
To find out discharge patterns ofa particular drainage basin
Help predict flooding events,therefore influence implementation of flood prevention measures
Why do we use hydrographs?
Hydrographs
Record of River Discharge over a period of time
The discharge of a river is the volume of water which flows through it in a given time. It is usually measured in cubic meters per second
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
char
ge (
m3/s
)
Base flow
Through flow
Overland flowR
isin
g lim
b Recession lim
b
Basin lag time
mm4
3
2
Peak flow
Flood H
ydro
graph
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
charg
e (
m3/s
)
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
charg
e (
m3/s
)
mm4
3
2
Rainfall shown in
mm, as a bar graph
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
charg
e (
m3/s
)
mm4
3
2
Discharge in m3/s, as a line graph
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
charg
e (
m3/s
)
Ris
ing
lim
b
mm4
3
2
Rising limbThe rising
flood water in the river
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
charg
e (
m3/s
)
Ris
ing
limb
mm4
3
2
Peak flow
Peak flowMaximum
discharge in the river
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
charg
e (
m3/s
)
Ris
ing
limb
Recession
limb
mm4
3
2
Peak flow
Recession limbFalling flood water in the
river
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
charg
e (
m3/s
)
Ris
ing
limb
Recession
limb
Basin lag time
mm4
3
2
Peak flow
Basin lag timeTime
difference between
the peak of the rain
storm and the peak
flow of the river
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
charg
e (
m3/s
)
Base flow
Ris
ing
limb
Recession
limb
Basin lag time
mm4
3
2
Peak flow
Base flowNormal
discharge of the river
0 12 24 36 48 30 72
Hours from start of rain storm
3
2
1
Dis
charg
e (
m3/s
)
Base flow
Through flow
Overland flowR
isin
g
limb
Recession
limb
Basin lag time
mm4
3
2
Peak flow
Overland flow
Through flow
+
=Storm Flow
Volume of water
reaching the river from
surface run off
Overland flow
Through flow
Volume of water reaching the
river through the soil and
underlying rock layers
Factors influencingHydrographs
• Area
• Shape
• Slope
• Rock Type
• Soil
• Land Use
• Drainage Density
• Precipitation / Temp
• Tidal Conditions
When interpreting hydrographs all factors must be considered together
When interpreting hydrographs all factors must
be considered together
Here are some theoretical interpretations of influencing
factors
BUT…
Area
Large basins receive more precipitation than small basins therefore have larger runoff
Larger size means longer lag time as water has a longer distance to travel to reach the trunk river
Area Rock Type Drainage Density
Shape SoilPrecipitation / Temp
Slope Land Use
Shape
Elongated basin will produce a lower peak flow and longer lag time than a circular one of the same size
Area Rock Type Drainage Density
Shape SoilPrecipitation / Temp
Slope Land Use
Slope
Channel flow can be faster down a steep slope therefore steeper rising limb and shorter lag time
Area Rock Type Drainage Density
Shape SoilPrecipitation / Temp
Slope Land Use
Rock Type
Permeable rocks mean rapid infiltration and little overland flow therefore shallow rising limb
Area Rock Type Drainage Density
Shape SoilPrecipitation / Temp
Slope Land Use
Soil
Infiltration is generally greater on thick soil, although less porous soils eg. clay act as impermeable layers
The more infiltration occurs the longer the lag time and shallower the rising limb
Area Rock Type Drainage Density
Shape SoilPrecipitation / Temp
Slope Land Use
Land Use
Urbanisation - concrete and tarmac form impermeable surfaces, creating a steep rising limb and shortening the time lag
Afforestation - intercepts the precipitation, creating a shallow rising limb and lengthening the time lag
Area Rock Type Drainage Density
Shape SoilPrecipitation / Temp
Slope Land Use
Drainage Density
A higher density will allow rapid overland flow
Area Rock Type Drainage Density
Shape SoilPrecipitation / Temp
Slope Land Use
Precipitation & Temperature
Short intense rainstorms can produce rapid overland flow and steep rising limb
If there have been extreme temperatures, the ground can be hard (either baked or frozen) causing rapid surface run off
Snow on the ground can act as a store producing a long lag time and shallow rising limb. Once a thaw sets in the rising limb will become steep
Area Rock Type Drainage Density
Shape SoilPrecipitation / Temp
Slope Land Use
The size, shape and relief of the basin are important controls. Water takes longer to reach the trunk stream in a large, round basin than in does in a small, narrow one.
Where gradients are steep, water runs off faster, reaches the river more quickly and causes a steep rising limb. Prolonged heavy rain causes more overland flow than light drizzly rain.
Areas of permeable rocks and soil allow more infiltration and so less surface run off.
The way in which the land is used will also have an influence on the hydrograph – vegetation intercepts precipitation and allows evaporation to take place directly into the atmosphere so reducing the amount of water available for overland flow while the large number of impermeable surfaces in urban areas encourages run off into gutters and drains carrying water quickly to the nearest river.